In recent years, digital devices such as a digital camera and a digital video camera are becoming lighter, thinner, shorter, and smaller and semiconductor devices in which many electronic components are mounted in a smaller space are used. A stacked type semiconductor device in which a plurality of semiconductor devices are stacked attracts attention. The stacked type semiconductor device is formed by bonding a semiconductor package by a joint material such as solder onto another semiconductor package in which a semiconductor element is mounted on an interposer.
The mainstream of semiconductor devices is a semiconductor package of BGA (Ball Grid Array) in which solder balls are used as joint materials which are external connection electrode. Among the BGAs, a semiconductor package, such as a CSP (Chip Size Package), which includes an interposer slightly larger than or equal to a semiconductor element, is increasingly employed.
Generally, thermal deformation such as warping occurs in the interposer due to heat generation when the semiconductor element operates. At this time, the solder used as the joint material is the softest of components of the semiconductor device and a printed wiring board on which the semiconductor device is mounted, so that the solder has a function to absorb a difference of coefficient of thermal expansion between the interposers or between the interposer and the printed wiring board. At this time, shear stress is concentrated to the solder.
When the semiconductor device is mounted on the printed wiring board, the shear stress generated in the solder, which is the joint material, is evaluated by a strain value obtained by dividing a difference between the amount of thermal deformation of the interposer of the semiconductor device and the amount of thermal deformation of the printed wiring board by the height of the solder, which is the joint material. When the semiconductor device is a stacked type semiconductor device, the shear stress is evaluated by a strain value obtained by dividing a difference between the amount of thermal deformation of the interposer of an upper semiconductor device and the amount of thermal deformation of the interposer of a lower semiconductor device by the height of the solder which is the joint material. The larger the strain value, the more the shear stress is concentrated and the easier a bonding portion is broken. Therefore, the smaller the diameter of the solder ball is set to in order to further downsize the semiconductor device, the larger the strain value and the easier the solder ball, which is the bonding portion, is broken.
In a stacked type semiconductor device, an interposer is further stacked and coupled onto another interposer on which a semiconductor element is mounted, so that a height higher than a height of the semiconductor element mounted on the lower interposer is required for the bonding portion (external connection electrode) connecting the upper and the lower interposers together. Therefore, when the diameter of the solder ball, which is the bonding portion, is simply reduced in order to downsize the semiconductor device, the height of the bonding portion cannot be higher than or equal to the height of the semiconductor element mounted on the lower interposer.
PTL 1 describes that a cylindrical metal post is formed on an electrode pad of the lower interposer and the solder ball is disposed on the metal post in order to ensure the height of the bonding portion.
However, in the case of the configuration described in PTL 1, when the diameter of the solder ball is decreased, the strain value described above increases and cracks tend to occur in the interface between the solder ball and the metal post. In particular, a shear stress is concentrated on the interface between the solder ball and the metal post where the physical property of materials drastically changes, so that there is a problem that long term reliability cannot be ensured.